The removal of a sorbable component from a gas or vapor stream by flowing such stream through a body of adsorbent material is a fundamental engineering practice. One type of sorbable components which are desirable to remove from a gas stream is volatile organic compounds VOCs.
VOCs are formed in large quantities but at relatively low concentrations from gas turbines, cogeneration plants, petrochemical plants, and in many industrial processes where waste gases contain such materials as vapors of various solvents, inks, paints, and so forth, VOCs contain not only hydrocarbons--saturated, unsaturated, and aromatic--but also contain oxygenated materials such as alcohols, esters, ethers, and acids, nitrogen containing compounds (principally amines), sulfur containing materials (mercaptans and thioethers) and halogen-containing materials, especially chlorine-substituted hydrocarbons but also organic fluorides and bromides. The presence of these VOCs in the gas stream can present a health risk or cause the gas stream to have an unpleasant odor.
The widespread use of solvents in industrial applications has resulted in increased emissions of VOCs into the atmosphere, giving rise to environmental concerns and prompting stricter legislative controls on such emissions. As a consequence, manufacturers of pharmaceuticals, coated products, textiles, and polymer composites and foams, as well as hydrocarbon producers and distributors, face a dilemma in removing VOCs from process gas streams in that, owing to rising energy prices, recovery costs are very often higher than the value of the VOCs recovered, even in light of rising solvent prices. This dilemma has led to inquiries into more profitable methods of removing condensable organic vapors from process gas streams.
Conventional adsorption systems for solvent recovery from humid air typically are operated until the solvent concentration in the outlet gas stream reaches a detectable preset breakthrough level whereupon the gas flow to the adsorber is stopped. The adsorbent bed then contains solvent, other condensable organic contaminants, and some amount of water which depends on the inlet relative humidity of the solvent laden gas stream. At this point, present-day techniques involve the introduction of steam, either saturated or superheated, which displaces the solvent from the adsorbent to produce a solvent/water mixture upon condensation. Typically two adsorber beds are used, where one is adsorbing while the other bed undergoes regeneration. More recent technology for regenerating and recovering solvent from adsorbent beds involves the use of inert gases (through for some solvents, air also can be used) and low temperature condensation of the solvent from the regenerating gas.
Of those techniques for removing VOCs from a gas stream by adsorption, the most common method is exemplified in U.S. Pat. No. 4,402,717 to Izumo et al. In Izumo et al., an apparatus for removing moisture and odors from a gas stream comprises a cylindrical honeycomb structure made from corrugated paper, uniformly coated with an adsorbent and formed in the shape of a disk or wheel. The multiplicity of adsorbent-coated parallel flow passages formed by the corrugations in the paper serve as gas passage ways which are separated as a zone for the removal of water and odor causing components in the gas, and as a zone for the regeneration of the adsorbent. The zones for removal and regeneration are continuously shiftable as the wheel is rotated circumferentially about its centerline.
The coated wheel units suffer many disadvantages. They require a large physical space to accommodate the enclosure for the wheel having the multiple removal and regeneration zones, and the associated gas transfer equipment (fans and blowers). The adsorbent-coated paper has limited range of humidity and temperature within which it can maintain its structural integrity. This failure also limits the regeneration medium to dry, moderate temperature gases and air. The contact between the adsorbent and the gas stream and hence the adsorbent capacity for the VOCs is limited to the very thin layers of adsorbent on the surface of the paper. This feature also limits the ultimate life of the adsorbent wheel, resulting in frequent wheel replacement. A further problem of the wheel system often occurs in treating fluids containing VOCs which can permanently poison the surface of the adsorbent either by blocking the pores as with compounds such as paint pigments, or by the inability of the desorption step to remove those poisons.
An adsorber module is sought which does not have the size limitations of the adsorbent wheel and has an increased adsorbent capacity for the removal of VOCs from gas streams.
It is a further object of this invention to provide an adsorbent module which has improved structural integrity which permits the use of saturated or superheated steam to regenerate the adsorbent and recover VOCs.
It is a still further object of this invention to provide an adsorbent module which is mechanically simpler to operate and is less costly to construct and operate.